1. Field of the Invention
This invention relates to a method for anisotropic plasma etching of substrates and to an apparatus for implementing the method, as well as to electronic devices manufactured in accordance with the method.
2. Background of the Related Art
Methods for anisotropic plasma etching, particularly for anisotropic plasma etching of silicon, distinguish themselves in that the structures to be produced can be worked into the substrate practically independently of the crystal orientation of the substrate. Preferred fields of application are micromachining and sensor technology, where structures having a high aspect ratio, i.e., a high structural height-to-width ratio, or having narrow grooves and vertical walls are to be produced, for example, for oscillating structures, capacitative or resonant sensors, electrostatic actuators, etc. Further fields of application include microelectronics, where so-called trenched grooves are required for component insulation or collector contact, and storage cells in general. Due to the production of narrow grooves by anisotropic plasma etching, a significant amount of chip area can be saved.
The RIE (reactive ion etching) processes which are commonly used for anisotropic silicon plasma etching employ the less reactive halogens chlorine or bromine, which are used either directly or are released from corresponding compounds, for example, CF.sub.3 BR, CCl.sub.4, CF.sub.2 Cl.sub.2, or CF.sub.3 Cl, in the plasma, as well as relatively high energy ions, for example, 100 eV and higher. The resulting ion bombardment onto the etching ground, i.e., the area to be etched, initiates the reaction of adsorbed halogen radicals with the silicon to be depleted, while the spontaneous reaction on the side walls of the etched structures, which are not exposed to any direct influence by ions, is minor with these less reactive halogens chlorine and bromine.
However, there are a multitude of problems connected with processes employing chlorine and/or bromine from the equipment and process technology points of view because these gases are extremely corrosive and are often poisonous and/or carcinogenic. Furthermore, they tend to produce a so-called undesirable "black silicon" on the etched area and exhibit low mask selectivity, i.e., masking material and substrate exhibit comparable etching rates. Moreover, these FCKW (fluorinated and chlorinated hydrocarbons) and/or FBrKW (fluorinated and brominated hydrocarbons) substances will not be available in the near future.
Processes employing a fluorine-based chemical, which partially overcomes these disadvantages, produce significantly greater silicon conversion rates and are substantially less critical with respect to process and equipment technology. However, fluorine-based chemicals exhibit an inherent isotropic etching characteristic, which means that the spontaneous etching reaction of fluorine radicals in relation to silicon is so strong that the side walls of etched structures are also significantly attacked and significant undercutting occurs. Due to the difficulty of controlling plasma chemistry, attempts to date have failed to produce, in a controlled manner, both F-radicals which affect etching and radicals of polymer-forming monomers, and to simultaneously maintain them for a sufficiently long time period in the plasma to allow polymer formation and coating of the side walls to protect the side walls of the etched structures from attack by etching while the area to be etched is attacked by the etching species. The coexistence of fluorine radicals and unsaturated monomers is nearly impossible to attain at low excitation densities. Moreover, the etching rates that can be attained in this manner are very low, so that the problem of mask material depletion which is too pronounced in relation to the silicon conversion rate, also remains unsolved.
It is therefore the object of the present invention to provide an anisotropic plasma etching method which overcomes the above disadvantages of prior art methods, an apparatus for implementing the method, and electronic devices which are advantageously provided with etched structures having a high aspect ratio and one of narrow or deep trenches.